This invention relates to novel photosensitive compositions. It more particularly refers to polyester-polyether block polymer type photosensitive compositions which are photopolymerizable by the action of actinic light and which are useful in the preparation of relief images, especially relief printing plates.
Unsaturated polyester type photosensitive compositions are already disclosed in, for example, U.S. Pat. No. 2,760,863, and Japanese Pat. Nos. 542,045 and 599,101. Image making articles such as relief plates may be produced by forming a layer of the photosensitive compositions of a desired thickness on a suitable base, exposing the layer to actinic light through, for example, a photographic negative film to photopolymerize the image areas and washing out the non-exposed areas. Relief plates thus obtained may be used as relief printing plates, dry offset printing plates, displays and name plates.
According to these Japanese patents relief printing plates having desired properties can be obtained by suitably selecting the average molecular weight of unsaturated polyesters, the number of ether bonds in the unsaturated polyesters and the type of crosslinking agents. However, with the progress in printing technique printing plates are required to have still higher impact resilience, tear strength, tensile strength, elongation and printing resistance and it has been found difficult to meet these requirements by these methods.
It is known to obtain rubber elastomers by introducing a polyether as a soft segment between two double bonds. It is also known to use a polyether or polyester as the soft segment in the molecular chain of polyurethanes. However, prepolymers containing a polyether alone as the soft segment between two double bonds in the molecular chain generally produce relief printing plates having low tensile strength and tear strength and consequently insufficient printing resistance. On the other hand the use of prepolymers containing a polyester alone as the soft segment between two double bonds in the molecular chain slightly improves the tensile strength of relief printing plates as compared with the prepolymers containing a polyether alone as the soft segment but the impact resilience is low and the lack of uniformity in inking and the "fatigue" of plates or deformation of relief images can easily take place. Thus, these defects are fatal especially for flexographic printing plates which require high elastic recovery.
In addition, it is necessary that unexposed portions of photosensitive compositions can be readily and easily removed with a solvent liquid. This washing-out property strongly influences the resolution of relief printing plates. The prepolymers containing a polyester alone or a polyether alone as the soft segment do not provide sufficient resolution.